if you want a good song that's long enough and that has a nice flow to it, check out Explosions in the Sky; they generally have really long songs (6-10+ minutes) that have such good flow. It might not be the type of song you're looking to use tho, because Explosions in the Sky is pretty much all instrumental, but they work good for long videos, or even short ones too. Just a thought/suggestion though, don't take it if you already have something else in mind...